Automatic focus control for a camera

ABSTRACT

A focus control for automatically maintaining the focus distance, i.e., lens-to-photoplate distance, of a camera within small tolerances utilizes gas-thrust bearings. The photoplate is mounted between two gas-thrust bearings called a lens bearing and a plate bearing. The lens bearing is very stiff and exerts a force which varies significantly with the float height; whereas, the plate bearing is very soft and exerts a force which is essentially independent of float height. Photoplate surface contours or irregularities are accommodated by a change in float height of the plate bearing while a substantially constant lensto-photoplate surface distance is maintained by the stiff lens bearing.

United States Patent Alles etal.

[54] AUTOMATIC FOCUS CONTROL FOR A CAMERA [72] Inventors: David ShepardAlles; John William Elek, both of Wescoesville; Benjamin Edward Nevis,Bethlehem; Wallace Albert Schlegel, Bath, all of Pa.

[73] Assignee: Bell Telephone Laboratories, Incorporated,

Murray Hill, NJ.

[22] Filed: Jan. 28, 1970 [21] Appl. No.: 6,438

[52] U.S.Cl

..95/4s, a so/25s ..G03l 3/00 Field ofSearch ..350/252, 255; 95/45 56]References Cited UNITED STATES PATENTS 3,081,682 3/1963 l(houry ..9s/4 sCONTROLLED COMPRESSED AIR [451 Jan. 25, 1972 Attorney-R. J. Guenther andEdwin H. Cave 57 ABSTRACT A focus control for automatically maintainingthe focus distance, i.e., lens-to-photoplate distance, of a camerawithin small tolerances utilizes gas-thrust bearings. The photoplate ismounted between two gas-thrust bearings called a lens bearing and aplate bearing. The lens bearing is very stiff and exerts a force whichvaries significantly with the float height; whereas, the plate bearingis very soft and exerts a force which is essentially independent offloat height. Photoplate surface contours or irregularities areaccommodated by a change in float height of the plate bearing while asubstantially constant lens-tophotoplate surface distance is maintainedby the stiff lens bearing.

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FROM CONTROLLED CONTROLLED COMPRESSED COMPRESSED AIR AIR 4 P2 CONTROLLED24 42 PNEUMATIC PRESSURE +1,

LENS BEARING PLATE BEARING LOA D P P, a P ARE CONSTANT I l I l l l FLOATHEIGHT /N [/5 N TORS By LEGEL A TTOR/VEV 1. Field of the Invention Thisinvention relates to camera focus control means and, more particularly,to focus controls suitable for use in industrial cameras such asstep-and-repeat cameras.

2. Description of Prior Art There are numerous applications forprecision cameras. Among these applications is the making of integratedcircuit masks wherein the camera is known as a step-and-repeat camera.In this particular application, in order to. produce repeatable, sharplydefined, and equally sized circuit mask images on the photosensitivesurface of the photographic plate, it is necessary to maintain thedistance between the camera lens and the surface of the photoplate towithin plus or minus 0.4 microns. The surface of the photographic plateor photoplate may deviate from a planar surface by as much as 0.5 to 1.0microns per inch of distance along the plate. Thus,

as the camera steps from position to form the mask images on thephotoplate, it would be very desirable to have a focus control whichwould automatically maintain a constant lens-tophotoplate surfacedistance by compensating for such deviations from a planar surface.

Some cameras presently used for such applications as described for thestep-and-repeat camera utilizes screw-type adjustments or other types ofmechanical drives to control the lens-to-photoplate surface distance,i.e., the focus distance. The focus distance may be measured byapparatus such as an interferometer and used to control the mechanicaldrives. However, these mechanical control means are not completelysatisfactory because of such factors as the speed of response and thehystersis effects which adversely affect repeatability. Gas-thrustbearings or air bearings have been previously used in cameras; however,there has been no known use of a combination of air bearings havingspecified characteristics to automatically control the camera focusdistance.

Therefore, it is an objective of this invention to improve the means formaintaining focus control in precision cameras.

Another objective is to enhance the ability of focus control means tomaintain a constant focus distance by automatically making adjustmentsfor surface irregularities in the photoplate.

SUMMARY OF THE INVENTION The foregoing objectives and others areachieved in accordance with the principles of the invention by theutilization of gas-thrust bearings having certain characteristics toadjustably support the photoplate.

- The movable photoplate is mounted between two gas-thrust bearings orair bearings called the lens bearing and the plate bearing. The weightof the photoplate is supported at an initial equilibrium positionbetween the bearings by a very soft spring and diaphragm arrangement.The lens bearing is rigidly maintained to fixed lens housing whereas theplate bearing may advantageously be flexibly maintained or supported bya pneumatic drive. The lens bearing is designed to be very stiff and tooperate on a portion of its load-float height characteristic curve wherethe load varies significantly with float height. The plate bearing isdesigned to be very soft and to operate on a portion of its load-floatheight characteristic curve where the load is essentially independent offloat height. Therefore, the plate bearing applies a substantiallyconstant force to one side of the photoplate regardless of variations inthe position of the photoplate. Thus surface irregularities in thephotoplate are automatically accommodated by a change in float height ofthe plate bearing while the photosensitive surface is forced to remain asubstantially constant distance from the lens by the stiff lens bearingin order to achieve the required force equilibrium.

DESCRIPTION OFTHE DRAWINGS The invention will be more readilycomprehended from the following detailed description and accompanyingdrawings in which:

FIG. 1 is a partly sectional schematic representation of the automaticfocus control of this invention;

FIG. 2 is an enlargedpartly sectional view of schematically representedgas-thrust bearings and a photoplate maintained therebetween; and

FIG. 3 is a series of load versus float height characteristic curves forgas-thrust bearings at some constant pressure.

DETAILED DESCRIPTION FIG. 1 showsa photoplatelO, which may be coatedwith an appropriate photosensitive substance such as an emulsion andupon which images are to be formed, fixedly mounted in a plate holder16. For example, photoplate 10 can be clamped to a shoulder {17 aroundthe inner periphery of holder '16 so that holder 16 moves withphotoplate 10. The plate holder 16 is connected to a base 30 through avery soft spring arrangement which might advantageously compriseparallel flexures or springs 18 and a flexible diaphragm 20. Althoughonly one side or portion of this mounting arrangement is shown in detailin FIG. 1, springsl 8 and diaphragm 20 advantageously extend full circleand thus support holder 1 6 around the entire periphery thereof. Theparallel flexures l8 and diaphragm 20 support the weight of photoplate10 and holder 16 at some designed or initial equilibrium position andprevent photoplate l0 fr om' rotating duringany movement thereof. Thelow pressure within the diaphragm 20, derived from a regulated pressuresource through conductor 36, may be varied to account for any variationin weight between different photoplates.

The photoplate 10 is mounted between two gas-thrust bearings designatedas the lens bearing 12 and the plate bearing 2 2. The lens bearing 12advantageously surrounds the camera lens 14 and is rigidly mounted tothe camera lens housing 26 which is attached to an appropriate frame 28.The plate bearing 22 is very flexibly mounted to and supported pneumaticdrive 24. The entire camera apparatus is supported by an appropriatesurface 32,such as a polished granite base.

As shown more fully in FIG. '2, the photoplate 10 is held at someinitial equilibrium position between lens bearing 12 and plate bearing22 by the soft diaphragm 20 and the parallel flexures] 8 previouslydescribed. This initial equilibrium position is adjustedso that thelens-to-photoplate surface distance, i.e., distance 34, is preciselythat required for the formation of precision images on the photoplate"10. At this point all forces on photoplate "10 are balanced.

As previously mentioned, the photoplate "10 has surface irregularitieswhich would cause an unacceptable variation in the lens-to-photoplatesurface distance as the camera steps or moves along the photoplate 10unless compensation is made therefor. The surface irregularities may bedue to such things as variations in emulsion thickness and the surfacecontours in the basic photoplatel 0. The irregularities may cause asmuch as 0.5 to 1.0 microns variation from a true planar surface per inchof travel along photoplate 10. Thus, in the case of the step-and-repeatcamera for making integrated circuit masks, the focus control systemmust compensate for these variations and hold the Iens-to-photoplatesurface distance constant to within plus or minus 0.4 microns. This isaccomplished by designing the lens bearing 12 and the plate bearing 22with the proper characieristics. v

FIG. 3 shows the load versus float height characteristic curve for thetwo gas-thrust bearings at some COIISt8f II pressure. These curves shownare one of a family of constant pressure characteri tic curves for eachbearing. The characteristic of a gas-thrust aring depends upon suchthings as a geometrical shape of as orifices and the operating pressurea is well known in the rt. In this invention the lens bearing 12 isdesigned to wo k on a portion of its characteristic curve where the loadvaries significantly with the float height, e.g., point A. Another wayof expressing this is to say that the lens bearing 12 operates at apoint where it has substantial stiffness. Thus, it is apparent from thecharacteristic curve that if the distance 34 between the lens bearing 12and photoplate 10, which is directly related to the criticallens-to-photoplate surface distance, decreases as shown in FIG. 2, theforce on the top of photoplate 10 will increase. This will tend to forcethe photoplate away from the bearing 12. Conversely, if the distancebetween lens bearing 12 and photoplate 10 increases, the force exertedby the lens bearing 12 decreases.

The plate bearing 22, on the other hand, is designed to operate at somepoint, such as point B, on its characteristic curve where the load isessentially independent of float height. That is, the stiffness of platebearing22 is negligible. Thus, plate bearing 22 will always exert anessentially constant force on photoplate l regardless of the distancebetween the bearing 22 and the photoplate 10. Therefore, it is apparentthat the two bearings -12 and 22, acting in conjunction, will tend tomaintain the photoplate l 0 in its equilibrium focus position. Forexample, assume that the photoplate has a surface irregularity whichdecreases the distances between lens bearing 12 and the photoplatelocation where the image is to be formed when the camera is moved fromone position to the next during the image making process. The forceexerted by lens bearing 12 on photoplate 10 will increase while theforce exerted by the plate bearing 22 on the photoplate 10 remainsconstant. The forces on photoplate 10 are no longer in equilibrium andthe photoplate 10 will be forced away from the lens bearing 12 until anequilibrium position is reached at which point the photoplate remainsstationary. This equilibrium position corresponds with the infocusposition of the photoplate 10. The bearings 12 and 22 are designed toapply a force only to a small area around the area of interest. Thus thephotoplate 10 will tend to always have its particular location orsection of interest at a given time in focus, i.e., thelens-tophotoplate surface distance will remain constant regardless ofirregularities in the photoplate '10 or emulsion surfaces.

In practice the requirement that plate bearing 22 have negligiblestiffness applies only to the situation where the plate bearing 22 isrigidly fastened to a support. In the embodiment illustrated in FIG. 1,wherein plate bearing 22 is flexibly supported by a relatively softpneumatic drive 24, the plate hearing 22 may have, and indeedadvantageously should have, some finite stiffness. However, thecombination of the soft pneumatic drive 24 with a constant pressure anda bearing 22 with a finite stiffness gives the same effect as a rigidlymounted bearing 22 of negligible stiffness. This results because thepneumatic drive plate bearing combination will follow the motion of thephotoplate l0 and maintain a constant pressure thereon.

The pressure to the bearings 12 and 22 and pneumatic drive 24 can easilybe controlled within the required tolerances. The pressure to bearings12 and 22 is derived from a regulated pressure source through conductors38 and 40 respectively. The pressure to drive 24 is from a regulatedsource through a conductor 42. It appears that pressure regulation towithin approximately one percent of a constant value is sufficient foreven the highly accurate focus control required in the stepand-repeatcamera. Such regulations can be obtained with commercially availablepressure regulators.

As has been previously discussed, the stiffness of the diaphragm 20 andthe parallel flexures 18 should be very small with respect to thestiffness of the lens bearing 12. A stiffness ratio of approximately 1/100 should be satisfactory for most applications. This insures that thediaphragm 20 and the paralthe weight of photo late 10 have beendetermined.

It is to be unders ood that the embodiment disclosed herein is merelyillustrative of the principles of the invention. Various modificationsthereto might be made by those skilled in the art without departing fromthe spirit and scope of the invention.

What is claimed is:

1. Apparatus for maintaining a constant focus distance between a cameralens and a surface of a photographic plate by compensating forvariations in said plate as said plate moves relative to said lenscomprising, in combination, first and second gas-thrust bearings havingopposed surfaces between which said plate is mounted, said first gasbearing applying a first force in a first direction to said plate, saidfirst force being substantially proportional to the distance betweensaid lens and said surface, said second gas bearing applying a secondforce to said plate in a second direction opposite to said firstdirection, said second force being substantially independent of saiddistance between said lens and said surface, said first and secondforces causing said plate to move between said opposed surfaces untilsaid first and second forces are in equilibrium said first and secondforces being in equilibrium when said distance between said lens andsaid surface equals said focus distance, whereby said focus distance isautomatically maintained at a constant value. 1

2. Apparatus in accordance with claim 1 including holding means formounting said plate between said opposed surfaces of said hearings in aninitial equilibrium position corresponding to said focus distance for aninitial portion of said plate, said holding means moving with said plateas said'first and second forces cause said plate to move to subsequentequilibrium positions corresponding to said focus distance forsubsequent portions of said plate, and means for supporting said holdingmeans from a fixed base so that said plate remains parallel to saidopposed surfaces during movement of said plate, said supporting meanshaving a relatively small stiffness.

3. Apparatus in accordance with claim 2 wherein said first bearing has astiffness at least times greater than said stiffness of said supportingmeans so that said supporting means has a negligible effect on saidmovement of said plate.

4. Apparatus in accordance with claim 2 wherein said supporting meanscomprises parallel flexure springs and an inflatable diaphragm.

5. Apparatus in accordance with claim 1 wherein said second bearing issupported by a pneumatic support having a relatively low stiffness.

6. Apparatus for maintaining a constant focus distance between a cameralens and a surface of a photographic plate during relative motion ofsaid lens with respect to said plate comprising, in combination, firstmeans for applying a first force to said plate, said first force beingproportional to the distance of said surface from said lens, and secondmeans for applying a second force to said plate in a direction oppositeof said first force, said second force being substantially independentof said distance of said surface from said lens said first and secondforces causing said plate to move between said first and second meansuntil said first and second forces are in equilibrium, said first andsecond forces being in equilibrium when said distance between said lensand said surface equals said focus distance, whereby said focus distanceis automatically maintained at a constant value.

1. Apparatus for maintaining a constant focus distance between a cameralens and a surface of a photographic plate by compensating forvariations in said plate as said plate moves relative to said lenscomprising, in combination, first and second gas-thrust bearings havingopposed surfaces between which said plate is mounted, said first gasbearing applying a first force in a first direction to said plate, saidfirst force being substantially proportional to the distance betweensaid lens and said surface, said second gas bearing applying a secondforce to said plate in a second direction opposite to said firstdirection, said second force being substantially independent of saiddistance between said lens and said surface, said first and secondforces causing said plate to move between said opposed surfaces untilsaid first and second forces are in equilibrium, said first and secondforces being in equilibrium when said distance between said lens andsaid surface equals said focus distance, whereby said focus distance isautomatically maintained at a constant value.
 2. Apparatus in accordancewith claim 1 including holding means for mounting said plate betweensaid opposed surfaces of said bearings in an initial equilibriumposition corresponding to said focus distance for an initial portion ofsaid plate, said holding means moving with said plate as said first andsecond forces cause said plate to move to subsequent equilibriumpositions corresponding to said focus distance for subsequent portionsof said plate, and means for supporting said holding means from a fixedbase so that said plate remains parallel to said opposed surfaces duringmovement of said plate, said supporting means having a relatively smallstiffness.
 3. Apparatus in accordance with claim 2 wherein said firstbearing has a stiffness at least 100 times greater than said stiffnessof said supporting means so that said supporting means has a negligibleeffect on said movement of said plate.
 4. Apparatus in accordance withclaim 2 wherein said supporting means comprises parallel flexure springsand an inflatable diaphragm.
 5. Apparatus in accordance with claim 1wherein said second bearing is supported by a pneumatic support having arelatively low stiffness.
 6. Apparatus for maintaining a constant focusdistance between a camera lens and a surface of a photographic plateduring relative motion of said lens with respect to said platecomprising, in combination, first means for applying a first force tosaid plate, said first force being proportional to the distance of saidsurface from said lens, and second means for applying a second force tosaid plate in a direction opposite of said first force, said secondforce being substantially independent of said distance of said surfacefrom said lens said first and second forces causing said plate to movebetween said first and second means until said first and second forcesare in equilibrium, said first and second forces being in equilibriumwhen said distance between said lens and said surface equals said focusdistance, whereby said focus distance is automatically maintained at aconstant value.